1. Field of the Invention
The field of the present invention relates in general to the fields of internal combustion engines and operation with variable types of fuels. More particularly, the field of the invention relates to adaptively controlling an internal combustion engine to operate on various fuels. The engine coupled with an adaptive control system performs in situ tests on the given fuel and based on these tests, configures the engine components to operate in a power cycle to take optimal advantage of the given fuel.
2. Background
Internal combustion engines are currently designed to run on a specific or particular type of fuel. This is a historical result based on manufacturing and assembly lines for economies of scale, aimed to keep vehicle price tag in the affordable range. Thus, engine designs were based on available fuels, which could be produced in mass quantities and lowest prices. This in turn resulted in internal combustion engines which burned a single type of fuel and only that particular fuel because the fuel combustion characteristics dictated that one and only that one combustion ignition method and engine component parameters static and dynamic, be used for maximum fuel efficiency at tolerable engine wear.
Current fuel sources and future fuel resources present the potential of many different combustible fuels, which cannot effectively be exploited by these single fuel design engines. Advanced computer technology is the difference that adds capability to engine designs. Engine designs that were not available when the basic internal combustion engine designs were cast. The rigid mechanical controls in early basic Otto and diesel engine from such components as camshafts and piston rod lengths have persisted and constrained engines to single fuels. While this is not an issue where the required fuel is cheap and abundant, consumers today are under large fuel price escalation and fluctuation pressures without recourse, as their engines depend on the one type of fuel set at engine manufacturing time. Even fuels that differ only slightly, high and low octane differ only by a few percent, bifurcate design into engines that can only burn high octane and engines that can only burn low octane. Running alternate fuels on current engine will greatly reduce engine life giving little or no engine performance.
Fuel prices generally go up uniformly despite a free economy and competition. Vehicles will need to be more adaptable to the kind of fuels they can use because structured, well defined distribution channels of various fuels will compete more efficiently and optimally where there is more consumer choice of fuels. Today's obsession with higher energy density or high performance fuels will have tradeoffs with available fuels and even low performance fuels. The future economics of fuels will have scarcities and distribution fluctuations. Consumers may need to look for fuel production locally available or even “grown” from what ever is proximately available as fuel candidates. Alternate fuel distribution stations and centers will need to enter the market. Hence there is a need for an internal combustion engine that is capable of ascertaining the type of fuel put in its tank in real-time and re-programming the engine operation parameters to accommodate that fuel or fuel blend accordingly.
Some automotive leaders are calling for a change to hydrogen fueled vehicles. California is contemplating a “hydrogen highway,” complete with 200 hydrogen filling stations, one every 20 miles. The public sector is pledging to pick most of the $100 million cost in California, the world's large car market. Vehicles will be transitioning to hydrogen fuel for the “hydrogen economy.” However, vehicle fuels used will vary for many political and economic reasons, with the vehicle engine being the perhaps only consumer controllable quantity. Hence there is a need for multi-fuel engines. This need is an engine that can run on available fuel and hydrogen if it is proximately available.
Hydrogen Powered Vehicles
Some auto industry experts proclaim hydrogen will be the next fuel used to power vehicles and some car manufactures have built model hydrogen fueled cars. These have come in two very different technologies. One way is a hydrogen fuel cell electric vehicle. The other method is to use hydrogen to fuel an internal combustion engine. Here the hydrogen is combusted with oxygen to generate power, hence turbo and super charging increases engine power and idle engine strokes wastes fuel. Innovations to the internal combustion engine will be directly applicable to hydrogen fueled internal combustion engines of the future. A new Ford model hydrogen fueled internal combustion engine is optimized to burn hydrogen through the use of high-compression pistons, fuel injectors designed specifically for hydrogen gas, a coil-on-plug ignition system, an electronic throttle, and new engine management software. This engine requires supercharging, which provides nearly 15 psi of boost on demand, but the engine is claimed to be up to 25 percent more fuel-efficient than a typical gasoline engine. Designing a spark ignition engine to burn hydrogen fuel has typically resulted in significantly lower power output, without supercharging. Even with supercharging, the hydrogen powered SI engine only delivers about the same power as its gasoline counterpart. This has been done at the expense of excluding gasoline combustion in the same engine, running hydrogen fuel exclusively. What are needed are engines that can exploit hydrogen fuel to greater potentials in power, efficiency and reduced pollution without foreclosing on the option of running on gasoline.
Hydrogen has a very wide combustion range, varying from 4 to 75 percent, hydrogen-fueled engines are able to use a wider range of air-fuel mixtures than gasoline engines, and can be run in the fuel-efficient “lean” regime without the complications of pre-ignition or “knock.” However, they have their own set of combustion challenges. What are needed are engines that can use hydrogen to attain their full potential, perhaps by combusting in an alternate mode like HCCI or blending with other fuels for alternate combustion characteristics.
Hydrogen Fuel Economy
Hydrogen fuels offer many advantages over current gas/diesel. However, a hydrogen engine will require hydrogen fuel. The currently guess is that it will be a 10 year transition to a hydrogen economy. Thus, hydrogen powered engines will only be sold where there is hydrogen fuel available. Hydrogen fuel stations are not likely to be economic unless there are sufficient hydrogen vehicles needing fuel. This chicken-egg paradigm inhibits the transition to a hydrogen economy. What is needed are engines which can run on currently available gas/diesel fuels as well as on hydrogen fuel, so as hydrogen fuel stations come to the market, they will have demand sufficient to make them a feasible alternative. What are needed are engines that can run on hydrogen and currently available or transitional fuels, to facilitate the market economics of hydrogen as well ad providing vehicle engines for transportation.
Hydrogen Economy Transition
As the worlds fuel consumption continues to accelerate, the supply of alternate sources and types of fuels will need to be exploited to meet demand. A hydrogen economy is in the foreseeable future but the transition from hydrocarbon to hydrogen fuels will be less painful if vehicle engines can run variable fuels. What is needed is an internal combustion engine that can use whatever fuel composition is available, be it octane, cetane, hydrogen, etc. What is needed are engines which will adapt and reconfigure themselves to whatever fuels are being put in their fuel tank.
Seeding Chemistry Approach to Alternate Fuels
Diesel engines and Homogeneous Charge Compression Ignition (HCCI) engines rely on auto-ignition for the initiation of combustion while in spark ignition (SI) engines auto-ignition leads to knock, which is a major constraint on efficiency and power. Where fuel and air are premixed as in the HCCI or SI engine some things are known. Practical fuels used in such engines are complex mixtures of hydrocarbons whose auto-ignition chemistry is not understood in detail. The auto-ignition quality of such a fuel has to be defined using an empirical approach. It can be best described by an Octane Index, OI defined as OI=(1−K)RON+K MON where RON and MON are the Research and Motor octane numbers respectively of the fuel while K depends only on engine design and operating conditions. The larger the OI, the greater is the resistance to auto-ignition. The RON and MON of any fuel can be determined by standard procedures that are based on comparing the fuel to mixtures of the two paraffinic fuels iso-octane and n-heptane for knocking behavior in a single cylinder test engine. The value of K is found empirically by ranking fuels of different chemistry for their auto-ignition behavior at a given operating condition. K can vary widely and can be negative or greater than unity depending on the pressure/temperature history of the air-fuel mixture. However K does not vary randomly but depends strongly on generic engine parameters such as the compression temperature at a fixed pressure in the engine and can be estimated from empirical results if such parameters can be predicted.
Calculations on a Homogeneous Charge Compression Ignition (HCCI) engine have been performed using zero-dimensional models. The simplest model compressed the gas to auto-ignition, using temperature and pressure at a certain crank angle position obtained from engine experiments.
It was found that calculations with good agreement could be accomplished, if using correct temperature, pressure and air-fuel mixture composition. However, the calculations proved to be extremely sensitive to even small variations in temperature. Further, natural gas engine calculations showed a high sensitivity to the contents of higher hydrocarbons such as ethane, propane and butanes. The validity of the kinetic mechanism was also a crucial factor. Due to the assumption of total homogeneity in the combustion chamber, a too rapid heat release was predicted.
A reaction mechanism for formaldehyde, methane and methanol was developed. The aim was to produce a combustion ignition mechanism of general characteristics, covering formaldehyde, methane, and possible methanol, giving correct species profiles for intermediate products. The mechanism was capable of accurately predicting ignition delays for formaldehyde and methane over a wide range, gave decent methanol auto-ignition prediction, and could further accurately predict the species profiles for formaldehyde but was not capable of calculating flame speeds for methane.
A semi-detailed reaction mechanism for Primary Reference Fuels, mixtures of iso-octane and n-heptane, has been developed. The predictions of ignition delay times shows a good agreement to experiments. Much has been done to study various fuels in internal combustion engines and many numerical models are available. What are needed are smart internal combustion engines that can use those models and empirical data, and engines can determine fuel combustion characteristics, engines that store their own design parameters and can re-configure themselves for a given particular fuel.
Fuel Gas and its Use in Natural Gas Engines
The main constituent of natural gas is methane; the Alaskan contains nearly ninety nine percent, and the Indonesian less than ninety. The remainders are ethane, propane, butane and propylene. The amount and combination of each component species in natural-gas oriented gaseous fuels, especially the ratio between propane and butane, change day by day. The constituent change of natural-gas oriented fuels results in the fluctuation of ignition characteristics for the internal combustion engines. The increase of butane content compared to propane would bring the spark-ignited natural gas engines to easier knocking tendency. Ignition characteristics of fuels are much more important for the HCCI engines, in which the ignition timing depends directly on the fuel ignition characteristics. Ignition timing itself dominates power output and fuel economy of the HCCI engines. The fuel species fluctuation in the supply lines adds another challenge to marketable HCCI engines. What are needed are HCCI engines that can adapt to alternate fuel species.
Basic ignition delay data are available, which recognize the anti-knock properties of supplied mixed fuels. Furthermore, some measures have been found to eliminate the fluctuation of ignition characteristics of fuels include an ignition-control concept utilizing the gaseous formaldehyde as an additive to realize premixed compression-ignition engines and to maintain stabilized operation of spark-ignited natural gas engines.
The formaldehyde is the most important intermediate during the pre-flame period up to the hot-flame establishment; closely related to the cool-flame appearance. It can be easily expected that formaldehyde is effective in promoting ignition of the non-cool-flame generating fuel such as methane; the other way around about the cool-flame generating fuels.
A present challenge in the premixed fuel HCCI engine is the lack of effective ignition control procedure. Some have proposed and confirmed that the formaldehyde as an efficacious additive into the mixture to realize wide-range premixed compression-ignition operation of the natural gas engine.
Measures to eliminate the fluctuation of ignition include lean air-fuel mixtures with various fuel/fuel ratios between methane and n-butane, supplied to a premixed HCCI engine with or without supplementary gaseous formaldehyde induction as an ignition control additive. The amount of formaldehyde at hundreds parts per million of total mixtures. In the no additive case the methane and butane function as the two fuels in the high/low-octane two-fuel premixed HCCI operation we proposed previously as another ignition control procedure. The formaldehyde addition to the methane-butane-air mixtures has given the engine desired and stable ignition timings controllable by the amount of formaldehyde to be added, almost independent on the fuel/fuel ratios between methane and butane. The efficacy of formaldehyde has been confirmed as an ignition control medium for the compression ignition of hydrocarbon-air mixtures.
The experimental evidence suggests that other fuels besides the traditional gas and diesel offer potential substitutes. What are needed are internal combustion engines that can take advantage of these blends, natural gas fuels, hydrogen and others. They must be capable of certain online real-time experimentation to establish a particular combustion state point and then engine re-configuration to conform with the necessary fuel characteristics.
Ignition Modes
Traditionally there have been four primary modes of operation for reciprocating internal combustion engines: spark ignition (SI), homogeneous charge compression ignition (HCCI), compression ignition (CI) and dual fuel compression ignition (DFCI). SI and CI engines have been commercially dominant due to the more simplistic and inexpensive control systems required for satisfactory operation. Today's vehicle technology using computers is formidable and while sophisticated and expensive by yesterdays standards, the computer control systems of today are more capable and more competitive. Hence what is needed is to make use of this by making internal combustion engines smarter, harnessing the basic hardware available in an internal combustion engine to solve more real world problems.
The Spark Ignition (SI) Engine
The combustion process usually starts in the center of the cylinder, after which the flame travels towards the cylinder walls. This means that SI combustion is characterized by a flame propagation process. Using a fixed air-fuel ratio means that load control is only possible by controlling the mass flow of air into the engine. The throttle that is used for this purpose gives rise to pumping losses and a reduction in efficiency; the major disadvantage of the SI engine is its low efficiency at part load. The compression ratio in Otto engines is limited by knock and can normally be found in the range from 8–12 contributing to the low efficiency.
Spark ignition (SI) engine operation involves ignition of a homogeneous or stratified mixture of air and readily vaporized high octane fuel, such as gasoline, using an electrical discharge (spark) from one or more ignition devices such as a sparkplug, located in the combustion chamber of the engine.
Ignition and combustion of the air-fuel mixture in SI engines is relatively slow, particularly at low loads, resulting in less than optimal thermal efficiency and fuel efficiency since only a portion of the fuel's energy is released at the point of maximum compression. Combustion of the air-fuel mixture begins at the sparkplug (under normal operating conditions). Since the flame has a single flame front, a finite period of time, which is dependent on many factors, is required for the flame (generated by the spark at the sparkplug) to propagate across the combustion chamber. The air-fuel mixture furthest from the sparkplug is ignited substantially later than the air-fuel mixture near the sparkplug. During flame propagation the pressure in the combustion chamber increases. The compressed air-fuel mixture furthest from the flame front is compressed to higher and higher values awaiting the flame. If the compression pressure and corresponding temperature of the air-fuel mixture awaiting the flame is sufficient, as well as the exposure time, the air-fuel mixture will auto-ignite before the flame reaches it. Auto-ignition of the air-fuel mixture results in very rapid rates of combustion generating high combustion pressures, rates of combustion pressure rise and combustion knock, which may cause engine damage depending on many factors. SI engines employ high octane fuels to minimize auto-ignition of the air-fuel mixture.
Homogeneous Charge Compression Ignition (HCCI)
Combining features from both spark-ignition and diesel engines, the Homogeneous Charge Compression Ignition (HCCI) engine is promising the high efficiency of a diesel engine with virtually no NOx or particulate emissions.
In the HCCI engine, fuel is homogeneously premixed with air, as in a spark-ignited engine, but with a high proportion of air to fuel. When the piston reaches its highest point, this lean fuel auto-ignites (spontaneously combusts) from compression heating, as in a diesel engine. But auto-ignition is what causes knock in a spark-ignited engine. Knock is undesirable in spark-ignited engines because it enhances heat transfer within the cylinder and may burn or damage the piston. But in an HCCI engine, with its high air-to-fuel ratio, knock does not damage the engine because the presence of excess air keeps the maximum temperature of the burned gases relatively low. When the danger of engine damage is eliminated, auto-ignition becomes a desirable mode of operation.
HCCI is very much fuel flexible, and like the Saab SVC engine, an HCCI engine could use a multitude of fuels. An HCCI engine could theoretically burn everything from diesel to natural gas simply by changing the compression ratio. However, diesel fuel would ignite with a compression ratio of 8:1, which is a problem because it reduces efficiency, while natural gas is difficult to ignite, igniting at a compression ratio of 19:1 or 20:1 but resulting in very high efficiency. It has been considered that variable-compression engine may be a way of controlling initiation of an HCCI cycle but no good way has found its way into an engine. Even the Saab uses spark ignition as a way of optimizing efficiency in the low-power range where most driving occurs. What is needed is an engine that can run in SI or HCCI modes to fully optimize the efficiency of an engine.
In an HCCI engine heat transfer in a multi-cylinder engine is harder to stabilize, making it more difficult in finding ignition points. This presents a cylinder unique feature not addressable with current means of inflexible cylinder controls. In a multi-cylinder engine, one cylinder that is even 5° C. hotter than the other one, which is not very appreciable, may be enough for it to burn way in advance of the other cylinders. Perhaps even the colder cylinders won't burn at all under those constrained compression ratio conditions. What is needed are ways to independently control cylinder unit compression ratios for those cylinder unique compression ratios with varied temperature ignition points.
DFCI Engines
For an efficient power stroke, auto-ignition must occur almost exactly at the point where the piston reaches its maximum height within the cylinder. Timing of auto-ignition is thus critical, but the HCCI engine gives up two timing control mechanisms: The start of ignition is not directly controlled by an external event such as the beginning of injection in the standard diesel or the sparking of the spark plug; and the heat release rate is not controlled by either the rate and duration of the fuel-injection process, as in the diesel engine, or by the turbulent flame propagation time, as in the spark-ignited engine. Detailed modeling of engines using a homogeneous charge of various fuels has shown that by knowing the precise conditions (fuel species, temperature, and density) at the start of compression, the beginning of combustion can be accurately predicted. But heretofore, the control problem is what keeps the HCCI out of the auto showroom, because these parameters are only established by computer codes and simulation. What are needed are smart internal combustion engines which can determine fuel characteristics in real-time and use that information to set the optimal engine control parameters and mode of engine operation to accommodate for that given unknown particular fuel. These would be coupled with computer controls and logic which, using tests and real-time analysis on the unknown incoming fuel, would determine the regions of best operation and reconfigure the engine parameters to accommodate that particular fuel with least engine damage and maximum fuel efficiency. What are needed are engines that can burn alternate fuels smart, efficiently and within tolerable engine wear parameters.
Compression Ignition (CI) or the Diesel Engine
Diesel engines operate at higher compression ratios (12–24) than SI engines. In this type of engines, varying the amount of Diesel fuel injected into the cylinder controls the load. Instead of ignition by a spark plug, the air-fuel mixture self-ignites due to compression. The processes that occur from the moment the liquid fuel leaves the injector nozzles until the fuel starts to burn are complicated; droplet formation, collisions, break-up, evaporation and vapor diffusion are some of the processes that take place. The rate of the combustion process is generally limited by these processes; a part of the air and fuel will be premixed and burn fast, but for the largest fraction of the fuel the time scale of evaporation, diffusion, etc. is larger than the chemical time scale. Liquid fuel that does only partially burn results in soot formation. Together with NOx, the emissions of soot characterize the diesel combustion process. For present engines, a trade-off between these two emissions is observed, which poses a major challenge to comply with future legislation for both emissions. The major advantages of the Diesel compared with the SI engine are the low pumping losses, due to the lack of a throttle, and a higher compression ratio, leading together to higher efficiency.
The advantages of SI over CI are lower cost because of higher production volumes. The advantages of CI over SI are higher fuel efficiency, higher power and durability. If the SI engine were subsumed in the CI engine, the CI engine would gain the lower cost due to higher volume. The disadvantages of SI are its low part-load efficiency and knock limited compression ratio. The disadvantages of CI over SI are noisier engines and high emissions of NOx and soot. Either way, the engines are designed around the method of fuel ignition for a particular fuel. What are needed are engine designs that configure themselves for the optimum use of available fuel. These would add utility and flexibility having the capability to run on gas, diesel, hydrogen etc and engine owners would not be constrained to a particular fuel burning vehicle.
CI engine operation is similar to HCCI operation in that a spark is not employed and auto-ignition of the fuel is accomplished by high compression pressures and temperatures. In addition, engine load and speed control is accomplished by controlling the quantity of fuel which enters the combustion chamber. The quantity of air supplied to the engine is not throttled to control engine load and speed as is done with SI engines.
However, unlike HCCI and SI engines, CI engines operate on low octane fuel, primarily diesel fuel. Low octane fuel such as diesel fuel is typically given a cetane value instead of an octane value. The cetane rating is the direct opposite of the octane rating since the cetane rating is a measure of a fuel's tendency towards auto-ignition. Higher cetane values indicate reduced resistance to auto-ignition and correspond to lower octane values. Commercial diesel fuel has a moderate cetane value in the range of 37 to 55 with most diesel fuel being sold with a cetane value of 40 to 47.
During CI operation only air is compressed during the majority of the compression process and as such very high compression pressures can be employed. Near the end of the compression process, injection of the fuel (under high pressure) into the combustion chamber is initiated. Ignition of the diesel fuel is not instantaneous upon injection into the combustion chamber. A period of time, referred to as the ignition delay period, exists between injection of the diesel fuel and the onset of combustion. The ignition delay period depends on numerous factors including engine speed, compression pressure and temperature, the quantity of diesel fuel injected and the cetane value of the diesel fuel. Ignition delay decreases with increasing compression pressure and temperature, increasing fuel cetane value, increasing quantity of fuel injected and decreasing engine speed.
The ignition delay period for CI engines typically ranges from 5 to 25 crankshaft degrees depending on the type of engine, engine speed/load, compression pressures and temperatures and the cetane value of the diesel fuel. During injection of the diesel fuel prior to ignition, the fuel begins to disperse and mix with the combustion air. If the ignition delay period is decreased, less air-fuel mixing occurs prior to ignition such that the combustion rate is reduced and rates of combustion pressure rise are low, minimizing combustion knock. If the ignition delay period is increased, more air-fuel mixing occurs prior to ignition such that the combustion rate is increased and rates of combustion pressure rise are high, generating significant combustion knock and engine stresses. At higher loads additional fuel is injected after ignition of the fuel in the combustion chamber. The rate of combustion of the additional fuel is controlled by the rate of injection. Although CI combustion occurs by autoignition, ignition of the fuel occurs with only partial mixing of the air and fuel, such that combustion is relatively slow in comparison to HCCI combustion in which the air and fuel are thoroughly mixed. In addition, ignition of the fuel occurs gradually since the fuel is injected into the combustion chamber over a finite period of time during the combustion process. As such CI combustion can generate high loads with peak combustion pressures, rates of combustion pressure rise and levels of combustion knock which are significantly lower than for typical HCCI combustion.
The CI combustion process generates higher thermal efficiency and fuel efficiency than the SI combustion process, due to higher compression pressures. In addition, less energy is required to induct air into the engine at low loads since the combustion air is not throttled to control load as is done with SI engines. CI engines can be operated satisfactorily throughout the range of low to high loads.
However, CI engines typically generate less power and engine speed than comparable displacement SI engines. The CI ignition and combustion processes are slow resulting in a reduction in combustion efficiency at moderate to high engine speeds. In addition, since the air and fuel are not thoroughly mixed prior to the combustion process, not all of the air is fully utilized for output power in a CI engine. The high combustion pressures generated in the CI engine and localized rich air-fuel mixtures also tend to generate higher NOx and particulate matter (PM) emissions than SI and HCCI engines.
HCCI methods also have disadvantages. In an HCCI multi-cylinder engine, any cylinder even 5° C. hotter than another one, not very much by engine standards, may be enough for that one cylinder to burn way ahead of the other cylinders, or perhaps the colder cylinders will not burn at all in accommodating the hot cylinder. Thus what is needed are ways to control cylinder compression ratios independently of other cylinders, so that small variations in compression ratio requirements due to small temperature differences across cylinders can be controlled without the constraint of uniform compression ratio for all cylinders.
Together with ignition timing concerns, the power output from an HCCI engine would be lower than an equal size diesel because the peak pressure limits how much power you can get from a given engine block. Because of the high peak pressure, an HCCI engine would need to reduce its output below a diesel of the same displacement. Thus what is needed is an engine which could run in CI or HCCI modes, depending on the type of fuel used and accommodating engine design.
Since turbulence plays no helpful role in HCCI combustion, ignition falls mainly into fuel chemical kinetics. A UC Berkeley team studying HCCI performance through computer models, divided the cylinder into 10 temperature zones, which were enough to predict maximum pressure, burn duration, indicated efficiency, and combustion efficiency. These results were verified experimentally on a single-cylinder engine operating on natural gas as data from the experiment and the computer model correlated closely.
For an effective HCCI engine, what is needed is a smart engine which can use the data developed from computer models and simulations providing empirical data to help improve the fuel combustion algorithms to converge to the best ignition temperature and pressure state sufficiently fast enough to adjust engine parameters to effectively control the engine. In a paper presented at Windsor Workshop 2000 Transportation Fuels ATF Engine Management Systems Session Toronto, ON, Jun. 6, 2000, by Jan-Roger Linna et. al, “The Holy Grail of Internal Combustion Engines . . . ” computer calculations for natural gas fuel combustion characteristics and experimental validation results were published. This kind of data on alternate fuel for combustion can be used to design smart internal combustion engines that can accommodate the burn characteristics of natural gas and it's family of characteristically related fuels.
Dual Fuel Engines
Dual fuel combustion ignition (DFCI) engines are typically low speed engines operated on a combination of natural gas and diesel fuel. Natural gas has historically been less expensive than diesel fuel and provides for cleaner combustion with reduced emissions. Engine load and speed control is primarily accomplished by controlling the combined quantity of natural gas and diesel fuel which are combusted with the air in the combustion chamber. In most applications the engine is operated as a conventional single fuel CI engine on diesel fuel at low loads. At higher loads natural gas is entrained into the air inducted into the combustion chamber, providing a homogeneous charge of air and natural gas to the combustion chamber. Auto-ignition of the natural gas by compression pressure and temperature is avoided by providing lean mixtures of air/natural gas. Lean air-fuel mixtures have a higher resistance to auto-ignition than stoichiometric air-fuel mixtures. Near the end of the compression process a small quantity of diesel fuel is injected into the combustion chamber through a pilot injector.
As previously discussed with regard to CI engines, ignition of the diesel fuel is not instantaneous upon injection into the combustion chamber. A period of time, referred to as the ignition delay period, exists between injection of the diesel fuel and the onset of combustion of the diesel fuel. The air/natural gas mixture contributes the majority of the energy to the combustion process at high loads. As such the quantity of pilot diesel fuel supplied for ignition purposes is relatively small in comparison to high load single fuel CI operation. The small quantity of pilot diesel fuel injected further increases the ignition delay period such that the pilot diesel fuel must be injected into the combustion chamber earlier in the compression process at maximum engine load and speed than would be the case for single fuel CI operation. Upon completion of the ignition delay period, ignition of the diesel fuel occurs and in turn causes ignition of the lean homogeneous air/natural gas mixture.
DFCI engines like CI diesel engines must operate at slower engine speeds than SI engines due to the lengthy ignition delay period of the diesel fuel and the slower combustion process. The ignition delay period can be reduced somewhat for CI engines as engine speed increases by increasing compression pressures through turbo-charging. However, increasing compression pressures to reduce ignition delay in a DFCI engine would cause unwanted instantaneous auto-ignition of the homogeneous air/natural gas mixture during the compression process, resulting in excessive combustion pressures and rates of combustion pressure rise. As such DFCI engine speeds tend to be more limited than for CI engines. Exhaust NOx and PM emissions at loads and speeds in which natural gas and diesel fuel are supplied to the combustion chamber are lower than for CI operation on diesel fuel alone.
The DFCI fuel control system is necessarily more complex than for SI and CI engines since the quantity and timing of two fuels entering the combustion chamber must be controlled. In addition, since instantaneous auto-ignition of the air/natural gas could result in excessive combustion pressures and rates of pressure rise, the control system must be capable of detecting auto-ignition and adjusting both fuel supplies accordingly to eliminate auto-ignition. Due to the complexity and corresponding cost of the DFCI natural gas/diesel fuel systems and limited range of engine operating speeds for efficient combustion, commercial applications have typically been applied to large low speed engines such as locomotive and stationary generator engines. Limited bus and truck fleet applications have also been commercialized.
What is needed are internal combustion engines which use methods to test for whether the fuel is optimally burned through SI, CI, or HCCI mode and to derive the characteristics of the fuel and apply control parameters and engine configuration which burns the given fuel efficiency and causes tolerable wear to the engine.
Detonation or Knock Background
U.S. Pat. No. 6,560,526—Matekunas, et al, Onboard misfire, partial-burn detection and spark-retard control using cylinder pressure sensing, is a recent invention entering the engine controls arena.
In '526 a ratio of the actual pressure to the motored pressure in the cylinder at one or more predetermined crank angles is used to estimate the fraction of fuel burned which, in turn, enables a determination of combustion failure in the cylinder cycle. Confirmation of said misfire or unacceptable partial burn leads to correction of engine operation by the controller and/or to a diagnosis of possible damage to the vehicle's catalytic converter.
Individual-cylinder pressure-based feedback has been shown to optimize engine operation using cylinder pressure as a fundamental combustion variable that is used to characterize the combustion process for each combustion event. Furthermore, it has been demonstrated that optimal engine control can be maintained by monitoring the pressure in each cylinder and using that information for feedback control of spark timing, exhaust gas re-circulation (EGR), air-fuel ratio (A/F), fuel balancing between cylinders, and combustion knock.
Matekunas proposes (see U.S. Pat. Nos. 4,621,603; 4,622,939 and 4,624,229) a methodology called “pressure-ratio management” which can be used in computer-based, closed-loop, engine-combustion control to better manage air-fuel ratio, including fuel balance between cylinders, ignition timing and EGR dilution, respectively. Matekunas' pressure-ratio management (PRM) involves computer-based engine controls and control algorithms which are facilitated by the availability of a production-viable, reliable cylinder-pressure sensor. The PRM methods require only a signal that has a linear relationship to the cylinder pressure without knowledge of either the gain or the offset of the cylinder pressure related signal. This provides the potential of applying sensors which need not be calibrated and which may measure pressure by means that are less direct than those sensors that must be exposed to the combustion gases in the engine cylinder. Such a sensor is a non-intrusive device called the “spark-plug boss” cylinder-pressure sensor as disclosed in U.S. Pat. No. 4,969,352.
PRM uses pressure data from one or more individual engine cylinders, at specified piston positions and corresponding known cylinder volumes. The fundamental basis for the data used is in the form of the ratio of the fired cylinder pressure and the pressure that would exist in the cylinder due to the presence of an air and fuel mixture if combustion did not occur. Pressure ratio is calculated for a piston position in terms of the current crank angle position.
However, the PRM is used to estimate the fraction of fuel burn to the exhaust re-circulation. Although PRM holds some advantages for controlling an engine using spark timing, exhaust gas re-circulation (EGR), air-fuel ratio (A/F), fuel balancing between cylinders, and combustion knock, it does so for tuning or optimizing purposes using EGR. Knock signals are simply treated without recognition for the phenomena complexity, adverse affects on engines or waveform analysis for other combustion characteristics. Cylinder valve timing and control is still constrained by a camshaft, and thus cylinder independent control is limited. Also, Matekunas addresses the harm potential to the catalytic converter, which is handled with more care. Temperature differences across the individual cylinders are not accounted for except for in air-fuel ratio. Furthermore, the main valve control is done via the traditional “hardwired” camshaft. What are needed are more versatile and direct methods to control individual cylinder mixture states and methods of extracting more information from cylinder fuel combustion, information which can be used in real-time to give the engine more versatility.
'526 teaches that PRM methods require only a signal that has a linear relationship to the cylinder pressure without knowledge of either the gain or the offset of the cylinder pressure related signal. That PRM pressure data is used in the form of the ratio of the fired cylinder pressure and the “motored pressure.” Thus, the PRM filters out most of the detonation knock signal. What are needed are methods of extracting more information from detonation signals for not only general knock reduction, which has been recognized and accomplished for current engines to an extent, but for algorithms which employ more sophisticated computer techniques to obtain additional advantages and promote alternate engine uses, functions with allow engines to not only have cleaner burns and protect the catalytic converter, but determine combustion characteristics by in situ experimentation and to re-configure themselves to operate in completely diverse air-fuel operating ranges to accommodate more than just one fuel operation, as well as providing cleaner burns and catalytic converter protection.
Knocking occurs when the flame from the spark plug does not consume the gases in the piston chamber fast enough or uniformly. The remaining “end gases” spontaneously combust, sending a damaging shock wave across the chamber. Engines operate most efficiently at the highest compression ratios, but that is precisely where knocking occurs also. Engine knock therefore sets an upper limit to the compression ratio at which a spark-ignited internal combustion engine can operate. Suppressing knock permits engines to operate at higher compression ratios and thus to achieve higher fuel efficiency and lower carbon dioxide emissions. In other ignition modes where fuel droplets of non-uniform size ignite out of concert with the flame front, knocking can also occur.
There are many methods for detecting and processing engine knock as it is a common engine problem. Wholesale computer oriented solutions such as data-driven fuel classification, time frequency analysis of combustion characteristics and computational fuel combustion kinetics are single study, single fuel limited. The sensor, detection and processing of knock signals is currently available. What is needed are engines capable of adjusting combustion parameters to ignite mixture in real-time, which minimize alternate secondary flame fronts and detonation waves causing knock. What are needed are engine controls systems which can detect, adequately recognize damage potential from the engine knock and alter engine combustion to an operating regime outside the damage zone, using available fuels. What is needed is to solve the vehicle to fuel type mismatch problem, the main problem of going to a hydrogen fuel economy where vehicles cannot always obtain hydrogen fuel and can rely on available gas or diesel sources where hydrogen fuel sources are not yet available.